Method of communication

ABSTRACT

A method of communication comprising determining whether to use distributing coding between a source (S), relay (R) and destination (D), based on a predetermined transmission rate; if the determination is positive, determining a forward error correction scheme using distributed Alamouti space-time coding, wherein the scheme is determined based on the predetermined transmission rate, a channel signal-to-noise ratio (SNR) and a network topology; relaying coded data from the S to the D using the determined forward error correction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a U.S. National Phase application under 35U.S.C. §371 of International Application No. PCT/SG2010/000045, filedFeb. 09, 2010, entitled A METHOD OF COMMUNICATION, which claims priorityto Singapore patent application number 200901050-5, filed Feb. 12, 2009.

FIELD OF THE INVENTION

The present invention relates to a method of communication, a relaystation, a base station, a communication network, a user equipment andan integrated circuit, and relates particularly though not solely todistributed rate setting and coding schemes.

BACKGROUND

The following abbreviations may be used in this specification:

-   MAC multiple access channels-   CMAC cooperative MAC-   MARC multiple access relay channels-   BRC broadcast relay channels-   DF decode-and-forward-   SNR signal to noise ratio-   FER frame error rate-   RSC recursive convolutional code-   LO local oscillator-   AWGN additive white Gaussian noise-   BPSK binary phase shift keying-   CC convolutional code-   BS base station-   UE user equipment-   OFDM orthogonal frequency division multiplex-   SCCP single carrier cyclic prefix-   OFDMA orthogonal frequency division multiple access-   SC-FDMA single-carrier frequency division multiple access-   DFT-Spread-OFDM discrete Fourier transform spread OFDM-   FEC forward error correction-   STBC space time block code-   LLR log-likelihood ratio-   IR incremental redundancy-   CP cyclic prefix-   TDMA time division multiple access-   RS relay station-   MIMO multiple input multiple output-   ACK acknowledgement-   NACK negative acknowledgement-   SDMA spatial division multiple access-   EXIT extrinsic information transfer-   S source-   R relay-   D destination-   CC(●) convolutional code with settings in parenthesis-   PCC(●) parallel concatenated code with settings in parenthesis-   P_(out)(●) probability of information outage-   I(●) mutual information-   γ_(AB) the SNR from A to B-   SNR_(AB) ^((N)) the SNR from A to B in the N-th time slot

SUMMARY OF THE INVENTION

In general terms the invention relates to combining distributed Alamoutispace-time coding with decode-and-forward cooperative relay strategyusing distributed rate-compatible error correction codes. This may haveone or more advantages such as:

-   1. a simple and/or systematic technique for designing a code for a    cooperative relay network may provide increased wireless capacity    and link quality;-   2. a coding technique for a cooperative relay network with a low    implementation complexity, for example using convolutional codes;    and/or-   3. a code design for a cooperative relay network which may have an    error performance that is close to the theoretical optimum.

In a first particular expression of the invention there is provided amethod of communication according to claim 1.

In a second particular expression of the invention there is provided aRS as claimed in claim 11.

In a third particular expression of the invention there is provided a RSas claimed in claim 12.

In a fourth particular expression of the invention there is provided aBS as claimed in claim 13.

In a fifth particular expression of the invention there is provided acommunication network as claimed in claim 14.

In a sixth particular expression of the invention there is provided a UEas claimed in claim 15.

In a seventh particular expression of the invention there is provided anIC as claimed in claim 16.

The invention may be implemented according to any of the embodiments inclaims 2 to 10.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more example embodiments of the invention will now be described,with reference to the following figures, in which:

FIG. 1 is an illustration of a simple 3-node relay channel modelaccording to a first example embodiment,

FIG. 2 is a graph of a Distributed Turbo coding at destination for arelay network with α=0.5, γ_(SD)=−3 dB and γ_(RD)=−2.1 dB,

FIG. 3 is a graph of an Enhanced Turbo code at relay for a relay networkwith α=0.75, γ_(SR)=0.1 dB, γ_(SD)=−3 dB and γ_(RD)=−1.2 dB,

FIG. 4 is a graph of an Enhanced Turbo Code at destination for a relaynetwork with α=0.75, γ_(SR)=0.1 dB, γ_(SD)=−3 dB and γ_(RD)=−1.2 dB,

FIG. 5 is a graph of a Distributed multiple Turbo code at relay for arelay network with α=0.75, γ_(SR)=−0.2 dB, γ_(SD)=−3 dB and γ_(RD)=−1.5dB,

FIG. 6 is a graph of a Distributed multiple Turbo code at destinationfor a relay network with α=0.75, γ_(SR)=−0.2 dB, γ_(SD)=−3 dB andγ_(RD)=−1.5 dB,

FIG. 7 is a graph of an outage and FER of distributed Turbo coding atdestination for a relay network with α=0.5,

FIG. 8 is a graph of an outage and FER of an enhanced Turbo coding andmultiple Turbo coding at destination for a relay network with α=0.75,

FIG. 9 is an illustration of a two-user CMAC system according to asecond example embodiment,

FIG. 10 is an illustration of a two-user MARC system according to athird example embodiment,

FIG. 11 is an illustration of a two-user BRC system according to a forthexample embodiment,

FIG. 12 is a graph of a Multiple Turbo code at destination for acooperative network with α₁=α₂=0.375, γ_(S) ₁ _(D)=γ_(S) ₂ _(D)=−3.3 dB,

FIG. 13 is a graph of an outage and FER of a multiple Turbo coding andan enhanced Turbo coding at destination for a cooperative network,

FIG. 14 is an illustration of the Network configurations,

FIG. 15 is an illustration of the CMAC: 1^(st) and 2^(nd) time slot,

FIG. 16 is an illustration of a CMAC: 3^(rd) time slot withoutcooperation,

FIG. 17 is an illustration of a CMAC: 3^(rd) time slot with cooperation,

FIG. 18 is a flowchart of the Joint network and channel encoding,

FIG. 19 is an illustration of the MARC: 1^(st) and 2^(nd) time slot,

FIG. 20 is an illustration of a MARC: 3^(rd) time slot withoutcooperation,

FIG. 21 is an illustration of a MARC: 3^(rd) time slot with cooperation,

FIG. 22 is an illustration of a BRC: 1^(st) time slot,

FIG. 23 is an illustration of a BRC: 2^(nd) time slot withoutcooperation,

FIG. 24 is an illustration of a BRC: 2^(nd) time slot with cooperation,and

FIG. 25 is a flowchart of the encoding structures for distributedcoding.

DETAILED DESCRIPTION

In the following one or more example embodiments are described includinga simple relay channel and three MAC network topologies in whichmultiple users need to exchange information sequences (packets) with abase station, namely, CMAC, MARC and BRC. In MARC, one or more dedicatedrelays are deployed to assist users' transmission to the base station,whereas in CMAC, no dedicated relays are available. In BRC, only oneuser is broadcasting with the help of one or more dedicated relays.

1. System Model

FIG. 1 shows a relay channel model 100, with a S, R and D. The channelcoefficients between S-R, S-D and R-D nodes are g₀, g₁, and g₂,respectively. We assume a quasi-static fading channel, where the channelcoherence time is considerably larger than the code word. The channelcoefficients are independent and identically-distributed randomvariables, which remain constant over the whole duration of thecodeword, given by Equation 1:

$\begin{matrix}{{g_{i} = \frac{h_{i}}{d_{i}^{\beta/2}}},} & (1)\end{matrix}$where h_(i) and d_(i) are the channel gains and distances between thetransmitter and receiver, respectively. The attenuation exponent is β(e.g., β=2 for free-space propagation).

The relay operates in half-duplex mode, where the transmitting andlistening modes cannot occur simultaneously. In the DF protocol, theblock of symbols with length n is split into two phases. In the firstphase, the relay is in listening mode and receives the signal from thesource. At the end of this phase, the relay decodes the sourceinformation message. The relay then switches to transmitting mode in thesecond phase and sends symbols to help the destination decode the sourcemessage. During the first phase, the signal received by the relay isgiven by Equation 2:y _(r,k) =g ₀ x _(s,k) +v _(k) , k=1, 2, . . . , αn,  (2)and the signal received by the destination is given by Equation 3:y _(k) =g ₁ x _(s,k) +w _(k) , k=1, 2, . . . , αn.  (3)where x_(s,k) denotes the codeword that is to be transmitted from thesource, v_(k) denotes the additive noise introduced in the channelbetween the source and the relay, and w_(k) denotes the additive noiseintroduced in the channel between the source and the destination. αdenotes the proportion of symbols that is to be devoted to the firstphase.

During the second phase (i.e. the relay transmitting phase), the signalreceived by the destination is given by Equation 4:y _(k) =g ₁ x _(s,k) +g ₂ x _(r,k) +w _(k) , k=αn+1, . . . , n.  (4)Here, x_(s)=[x_(s,1) . . . x_(s,n)]^(T) a is the source codeword, drawnfrom the code χ_(s). We assume that the symbols x_(r,k) transmitted bythe relay are from an auxiliary code x_(r) with length n. Only the last(1−α)n symbols of a codeword are effectively transmitted in the secondphase. In the first phase, the relay is idle because of the constraintsof half-duplex communication.

The noise variables v_(k)˜CN(0,σ_(v) ²) and w_(k)˜CN(0,σ_(w) ²) at therelay and destination, respectively, are mutually independent. v_(k) andw_(k) are complex variables and the notation ˜CN(●) denotes a complexGaussian distribution. We also impose the same per-symbol average powerconstraint for both the source and the relay in Equation 5:E[|x _(s,k)|² ]≦E _(s) and E[|x _(r,k)|² ]≦E _(s)  (5)where E_(s), denotes the symbol energy and E[ ] denotes an expectationoperation. The SNRs of the S-D and the S-R links are defined asγ=E_(s)/σ_(w) ² and {tilde over (γ)}=E_(s)/σ_(v) ², respectively. σ_(v)² and σ_(W) ² can be chosen such that σ_(v) ²=σ_(W) ².

2. Alamouti-DF Scheme

A DF protocol with the relay code χ_(r) such that the signal received atthe destination forms an Alamouti constellation, shall be referred to asthe Alamouti-DF scheme. Assuming that the relay can decode the signal,the signal transmitted by the relay at time k is given by Equation 6:

$\begin{matrix}{x_{r,k} = \left\{ \begin{matrix}{x_{s,{k + 1}}^{*},\mspace{14mu}{k = {{\alpha\; n} + 1}},{{\alpha\; n} + 3},\ldots} \\{{- x_{s,{k - 1}}^{*}},\mspace{14mu}{k = {{\alpha\; n} + 2}},{{\alpha\; n} + 4},{\ldots\;.}}\end{matrix} \right.} & (6)\end{matrix}$x_(s,k+1)* denotes the complex conjugate of x_(s,k+1). The signal seenby the destination for αn+1≦k≦n is an Alamouti constellation. Throughlinear processing of the received signal, the destination obtains thesufficient statistics for decoding, as are given by Equation 7:

$\begin{matrix}{{\overset{\sim}{y}}_{k} = \left\{ \begin{matrix}{{{g_{1}x_{s,k}} + w_{k}},} & {{k = 1},\ldots\mspace{14mu},{\alpha\; n}} \\{{{\sqrt{{g_{1}}^{2} + {g_{2}}^{2}}x_{s,k}} + {\overset{\sim}{w}}_{k}},} & {{k = {{\alpha\; n} + 1}},\ldots\mspace{14mu},n,}\end{matrix} \right.} & (7)\end{matrix}$where the statistical properties of {tilde over (w)}_(k) are identicalto those of w_(k). The mutual information per symbol at the destinationis given by Equation 8:I(γ,g ₁ ,g ₂)=αI(|g ₁|²γ)+(1−α)I((|g ₁|² +|g ₂|²)γ)  (8)On the other hand, if the source does not transmit during the secondphase, the mutual information is given by Equation 9:I(γ,g ₁ ,g ₂)=αI(|g ₁|²γ)+(1−α)I(|g ₂|²γ)  (9)

With a large n, the probability of a FER is the information outageprobability which is defined in Equation 10:P _(out)(γ,R)=Pr{I(γ,g ₁ ,g ₂)≦R},  (10)where R is the target transmission rate in bits per channel use. Forlarge SNR, the P_(out) is given in Equation 11:P _(out)(γ,R)˜κγ^(−d),  (11)where κ is the coding gain independent of γ and d is referred to as SNRexponent or diversity.

3. Distributed Coding FEC

A distributed turbo coding scheme may use a recursive convolutional code(RSC). When decoding the message at the relay, the interleaved messageis encoded with another RSC code. To further improve the decodingcapability at the relay, an enhanced turbo code scheme may be used.Instead of the RSC, a turbo code may be used at the source node in thedistributed turbo coding scheme. In addition to the systematic bits, thesource node transmits a punctured sequence of parity bits from the firstand second constituent encoders. The relay then transmits all thepunctured parity bits. The punctured turbo code has more parity bits atthe destination, resulting in an enhanced turbo code. Note that in allthe above mentioned schemes, systematic codes are used. The distributedturbo coding and the enhanced turbo code schemes may be combined toproduce a multiple turbo code at the destination. For the distributedmultiple turbo code scheme, the source also transmits using a turbocode. Instead of sending the punctured parity bits at the relay, theinterleaved message is encoded with another constituent code.

The coding techniques (such as but not limited to RSC, turbo codes ortheir corresponding constituent codes) may optionally employ puncturing.

In the following, we assume an AWGN channel (i.e., the channel gainh_(i)=1) where the SNR of source-destination channel is insufficient tosupport the desirable rate R.

The setup for the distributed turbo coding scheme is as follows. Thenon-systematic RSC C₁ at the source is CC(4/7) with R₁=1 and the RSC C₂at the relay is CC(7/5) with R₂=1. These codes are used together withBPSK modulation as shown in Equation 12:x _(s,k)={−√{square root over (E _(s))},√{square root over (E_(s))}},  (12)

producing an overall rate of R=1/2 bits per channel use for half-duplexmode with α=0.5. The SNR of the S-D channel is set as shown in Equation13:γ_(SD)=10 log₁₀(|g ₁|²γ)=−3.0 dB,  (13)

which may be sufficient for 1/2 bits per channel use. The relaytransmits the codeword from C₂, assuming the SNR of the S-R channelγ_(SR) is high enough for the relay to decode the message reliably. FIG.2 illustrates the EXIT chart for this distributed turbo coding schemewhen the SNR of the R-D channel is set as out in Equation 14:γ_(RD)=10 log₁₀(|g ₂|²γ)=−2.1 dB.  (14)

The average decoding trajectory is also included. A tunnel exists toallow for the convergence of iterative decoding towards a low errorrate. A rate of ½ bits per channel use is achievable for γ_(RD)=−2.6 dB.Hence, this code operates approximately 0.4 dB from the theoretic limit.

When γ_(SR) is low, the enhanced turbo code scheme is required for themessage to get across reliably to the relay. Again, we select our targetrate to be R=½ bits per channel use for half-duplex mode. Since γ_(SR)is low, we need to increase α to get the message across to the relay. Weselect α=0.75. The SNR for the S-D channel is set at γ_(SD)=−3.0 dB. Thecodeword sent by the source is formed by puncturing the turbo code whichis made up of constituent codes C₁ and C₂. The puncturing patterns forC₁ and C₂ are [1011] and [1110], respectively. Since the relay onlyreceives data in the first time slot, the relay would only see C₁ andC₂, which constitutes a rate 2/3 punctured turbo code. The EXIT chartfor the iterative decoding algorithm is given in FIG. 3. A tunnel existsto allow for convergence of iterative decoding towards a low error rateat γ_(SR)=0.1 dB. The SNR limit for a rate 2/3 is −0.7 dB. The relaythen transmits the punctured parity bits to the destination. FIG. 4illustrates the EXIT chart for the iterative decoding at thedestination. A tunnel exists to allow for convergence of iterativedecoding towards low error rate at γ_(RD)=−1.2 dB. The theoretical SNRlimit is −2.3 dB. Hence, this code is operating approximately 1.1 dBfrom the theoretic limit.

Instead of sending the punctured parity bits, the relay uses a encoderin the distributed multiple turbo code scheme. The encoder canoptionally use puncturing in its coding scheme. The constituent codesC₁, C₂ and C₃ are CC(4/7), CC(4/7) and CC(3), respectively. In FIG. 5, atunnel exists to allow for convergence of iterative decoding towards lowerror rate at γ_(SR)=−0.2 dB, which is lower than the enhanced turbocode scheme in FIG. 3. The convergence of iterative decoding towards lowerror rate is possible at γ_(RD)=−1.5 dB, as shown in FIG. 6. This codeis operating approximately 0.8 dB from the theoretical limit.

4. Alamouti-DF Scheme with Distributed Coding

According to an example embodiment, an Alamouti-DF scheme is used with adistributed turbo code, an enhanced turbo code and a multiple turbo codeschemes. The fading coefficients {h_(i)} are Rayleigh distributed asrepresented in Equation 15:p(h _(i))=2h _(i) e ^(−h) ^(i) ² , h _(i)≧0  (15)

FIG. 7 shows the outage probability and FER performance of anAlamouti-DF scheme with distributed turbo code. The setting of the relayis such that d₁ ^(−β)=1 and d₂ ^(−β)=1.23, where d₁ denotes the distancebetween the source and the destination and d₂ denotes the distancebetween the relay and the destination. The slot allocation is α=0.5 andthe target rate is R=0.5. With BPSK modulation, the outage probabilityhas a diversity of 2. The FER of the distributed turbo code (C₁ and C₂)is about 1 dB from the outage limit and has the same order of diversity.

FIG. 8 shows the outage probability and FER performance of anAlamouti-DF scheme with enhanced turbo code and multiple turbo code. Weset α=0.75, d₁ ^(−β)=1 and d₂ ^(−β)=1.51. The outage probability has adiversity of 2 only if R≦(1−α). With R=0.5, only a diversity of one maybe achieved. Similarly, the FER of the enhanced turbo code PCC(4/7+5/7)and the multiple turbo code PCC(3+4/7+4/7) is around 1 dB from theoutage limit with diversity one.

5. Rate Setting

According to the first example embodiment a node (for example the relay)acquires the channel state information, e.g., the SNR parameters, via,e.g., estimation based on preambles sent by the node, or feedback fromthe other nodes in the network. The transmission rate is set based onthe channel state information by using a rate setting algorithm.

Firstly a target rate is set for the user. If the channel qualityinformation shows that the target rate can be supported, then directlink transmission is used. If the target rate cannot be supported bydirect link transmission, then distributed coding is used.

For distributed coding, given the SNRs of the network, parameters may beselected to ensure reliable transmission is possible. For example, theslot duration for each phase of transmission should be minimized, e.g.,by using high-rate coded modulation, so as to improve the overallefficiency. The slot duration in some communication phases can beoptimized by, e.g., using an information theoretical approach to obtainthe rate region of the protocol adopted. This rate region provides aminimum SNR threshold which is required for operating at a certain rate.With these values, the minimum power to support the target rate can beobtained. If the SNR is below the minimum threshold, the target rate isreduced and the rate setting procedure starts all over again.

As discussed later a FEC approach is taken to achieve the selected rate.The rate information and the FEC parameters are transmitted to the nodesin the network and the nodes then start the transmission based on theset rate, code, and protocol.

6. Transmission Protocol

The transmission protocol for the relay network in FIG. 1 will now bedescribed. A Node S works in cooperation with a Node R to deliver itspackets to a Node D. In the 1^(st) time slot Node S transmits and Node Rreceives the coded bits and tried to decode its information. We considertwo distributed coding scheme: (i) incremental bits and (ii) jointnetwork and channel coding. Their encoding structures are shown in FIG.25. For incremental bits, Node S sends the codeword (c₁₁,c₂₁) during thefirst time slot. c₁₁ is the codeword produced by Encoder 1 and c₂₁ isthe codeword produced by Encoder 2 during the first time slot. Whenjoint network and channel coding is used, Node S sends the codeword(c₁,c₂). c₁ and c₂ respectively are the codewords produced by Encoder 1and Encoder 2. ACK/NACK information is sent out from node R to Node Sand D.

In the 2^(nd) time slot, if NACK is received from Node R, the sourcewill operate in anon-cooperative mode. If ACK is received, Node S and Dwill operate in a cooperative mode. In cooperative mode Node R sendseither (c₁₂,c₂₂) or (c₃) with the source using a STBC. c₁₂ and c₂₂ arethe codewords produced by Encoder 1 and Encoder 2 respectively duringthe 2^(nd) time slot while c₃ is the codeword produced by Encoder 3. Inthe non-cooperative configuration, Node S transmits additional codedbits during the 2^(nd) and last time slot for additional redundancy. Forincremental bits, S1 sends (c₁₂,c₂₂) while for joint network and channelcoding, S1 sends (c₃).

7. Encoding Scheme

Two types of distributed coding schemes are shown in FIG. 25. Firstlythe incremental bit encoders are rate 1 convolutional or recursiveconvolutional codes with appropriate puncturing patterns. The rate ofcodeword (c₁₁,c₂₁) is optimized for the SNR of the source to relaychannel, while the rate of codeword (c₁₁,c₁₂,c₂₁,c₂₂) is optimized forthe SNR of the relay to destination and the source to destinationchannel. Optimization of the code is done by pairing the extrinsicinformation transfer function of each of the component code, so that theSNR of the decoding threshold is minimized.

The joint network and channel coding encoders are rate 1 convolutionalor recursive convolutional codes with appropriate puncturing patterns.The rate of codeword (c₁,c₂) is optimized for the SNR of the source torelay channel, while the rate of codeword (c₁,c₂,c₃) is optimized forthe SNR of the relay to destination and the source to destinationchannel. Similarly, extrinsic information transfer functions are used tominimize the SNR of the decoding threshold.

With Incremental bits, in the cooperation phase, the source and therelay act like a virtual MIMO system and send the codeword (c₂₁,c₂₂)using a STBC. With joint network and channel coding, in the cooperationphase, the source and the relay act like a virtual MIMO system and sendcodeword (c₃) using a STBC.

8. Extension to Other Systems

The Alamouti-DF scheme with distributed coding for the relay network canalso be extended to other systems, like the second example embodimentCMAC shown in FIG. 9, the third example embodiment MARC shown in FIG. 10and the forth example embodiment BRC shown in FIG. 11. Orthogonalchannels can be assigned to each pair by using, e.g., TDMA, OFDMA,SC-FDMA, DFT-Spread-OFDM or SDMA.

8.1 CMAC Topology

We consider two users, S1 and S2, communicating with the BS usingOFDM/SCCP in FIG. 9. The cooperative distributed coding and Alamoutitransmission may use three time slots to complete one cooperation cycle.

8.8.1 Time Slot 1

S1 transmits a FEC-coded and OFDM/SCCP modulated sequences, where theFEC code rate is given in Equation 16:R ₁≦α₁ I(SNR_(S) ₁ _(S) ₂ ⁽¹⁾)  (16)

where α₁ is the fraction of time slot 1 and SNR_(S) ₁ _(S) ₂ ⁽¹⁾ is theSNR from S1 to S2. The coding scheme used in the transmission from S1may optionally employ puncturing.

8.1.2 Time Slot 2

S2 transmits FEC-coded and OFDM/SCCP modulated sequences, where the FECcode rate is given in Equation 17:R ₂≦α₂ I(SNR_(S) ₂ _(S) ₁ ⁽¹⁾)  (17)

where α₂ is the fraction of time slot 2 and SNR_(S) ₂ _(S) ₁ ⁽¹⁾ is theSNR from S2 to S1.

8.1.3 Time Slot 3

In this time slot, Alamouti-DF scheme with the enhanced turbo code andthe multiple turbo code schemes can be used. The procedure of encodingfor the enhanced turbo code scheme is the same as that for the relaynetwork illustrated in FIG. 1. As for the multiple turbo code scheme,each user interleaves the two information sequences separately, followedby feeding the sequences alternatively to an encoder.

S1 and S2 then transmit the sequence with distributed Alamouti STBC. TheAlamouti STBC provides diversity gain and reduces outage in a fadingchannel. The BS collects the received sequence, and computes the LLRbased STBC decoding for the IR. It then performs FEC decoding using allthe LLR information collected during the 3 time slots.

The rate region for time slot 3 for the enhanced turbo code scheme issimilar to that for the relay network. For the multiple turbo codescheme, the rate region is given by Equation 18:R ₁≦α₁ I(SNR_(S) ₁ _(D) ⁽¹⁾)+α₃ I(SNR_(S) ₁ _(D) ⁽²⁾+SNR_(S) ₂ _(D) ⁽²⁾)R ₂≦α₂ I(SNR_(S) ₂ _(D) ⁽¹⁾)+α₃ I(SNR_(S) ₁ _(D) ⁽²⁾+SNR_(S) ₂ _(D)⁽²⁾)  (18)R ₁ +R ₂≦α₁ I(SNR_(S) ₁ _(D) ⁽¹⁾)+α₁ I(SNR_(S) ₂ _(D) ⁽¹⁾)+α₃ I(SNR_(S)₁ _(D) ⁽²⁾+SNR_(S) ₂ _(D) ⁽²⁾)where α₃ is the fraction of time which slot 3 occupies,

${\sum\limits_{t = 1}^{3}\alpha_{t}} = 1.$The achievable rate of the system is given by the intersection of therate regions. Note that Users 1 and 2 do not have any new information tosend during this slot.

8.1.4 BS Processing

At the BS, iterative decoding is performed to decode the informationbits of Users 1 and 2, which is the same as the relay network if theenhanced turbo code scheme is used. For the multiple turbo codingscheme, iterative decoding is also used for the information sequences.

FIG. 12 illustrates the EXIT chart for the multiple turbo coding schemewhen γ_(S) ₁ _(D)=γ_(S) ₂ _(D)=−3.3 dB in an AWGN channel. γ_(S) ₁ _(D)and γ_(S) ₂ _(D) respectively denote the SNRs of the S1-D and the S2-Dlinks. The settings of the network is such that d_(S) ₁ _(D) ^(−β)=1 andd_(S) ₂ _(D) ^(−β)=1. d_(S) ₁ _(D) denotes the distance between S1 and Dwhile d_(S) ₂ _(D) denotes the distance between S2 and D. The slotallocation used is α₁=α₂=0.375 and the target rate is R₁=R₂=0.25. S1 andS2 use the multiple turbo code PCC(3+4/7+4/7). The upper bound curve1200 and the lower bound curve 1206 respectively illustrate theextrinsic log-ratios from C₁ and C₂ for the EXIT functionI(u⁽¹⁾;E⁽¹⁾(u₁)E⁽²⁾(u₁)) of S1, where u⁽¹⁾ denotes the information bitsfrom S1, while E⁽¹⁾(u₁) and E⁽¹⁾(u₂) denote the extrinsic LLR from theC₁ and C₂ of S1. Each vertical step in the EXIT chart corresponds to anactivation of the iterative decoding for S1, until convergence occurs.Similarly, each horizontal step corresponds to the activation of theiterative decoding for S2 until convergence occurs. The step curve 1202corresponds to the mutual information measured at the output of thedecoders under such an activation scheme. For lower complexity, weconsidered the activation of S1 (C₁−C₂−C₃) and S2 (C₁−C₂−C₃). The stepcurve 1204 corresponds to the mutual information measured at the outputof the decoders. The convergence of iterative decoding towards low errorrate is possible since a tunnel exists at −3.3 dB. The theoretic limitfor the target rates is −3.6 dB.

FIG. 13 shows the outage probability and FER performance. The outageprobability of the multiple turbo code has a diversity of 2, which ishigher that of the enhanced turbo code scheme, which has a diversityof 1. The multiple turbo code PCC(3+4/7+4/7) has an outage that isaround 0.3 dB from the outage limit.

8.1.5 Training for Channel Estimation

Normal training for time slot 1 and time slot 2 and orthogonal trainingsequence, e.g., the training sequences of [a a] for S1, [a −a] for S2.

8.1.6 Time and Frequency Synchronization

Conventional time and frequency synchronization can be used for timeslot 1 and time slot 2. For time slot 3, the two sequences should bealigned within the CP at BS so as to maintain subcarrier orthogonality.The two users should also use the same LO reference, e.g., the BS LOfrequency, for easier frequency synchronization at BS.

8.1.7 Transmission Protocol

For the CMAC network in FIG. 9, S1 and S2 work in cooperation to delivertheir packets to a common destination BS. In the 1^(st) time slot NodeS1 transmits, while Node S2 transmits in the 2^(nd) time slot. Each Snode receives the coded bits sent by its partner node and attempts todecode its partner information. The transmission scheme for these 2 timeslots is shown in FIG. 15. The codeword transmitted depends on whichdistributed coding scheme is used. For incremental bits, Node S1 sendsthe codeword (c₁₁,c₂₁). When joint network and channel-coding is used,Node S1 sends the codeword (c₁,c₂). If decoding is successful, then thisinformation will be relayed to the destination BS. ACKs/NACKsinformation is sent out from the source nodes to indicate whether theyare cooperating or not in the 3^(rd) time slot.

In the 3^(rd) time slot, cooperation occurs when both the sources sendACKs. Otherwise, the sources will operate in a non-cooperative mode. Inthe cooperative configuration, as shown in FIG. 17, Node S1 and S2encode both information bit streams either jointly or separately andtransmit them to the destination BS using a STBC. For separate encoding,the codeword (c₁₂,c₂₂,{tilde over (c)}₁₂,{tilde over (c)}₂₂) is sent byS1 and S2 using STBC. For joint network and channel encoding, as shownin FIG. 18, S1 and S2 send the codeword (c₃′) to the destination, alsousing STBC. c₁₂ and c₂₂ are the codewords sent by S1 while {tilde over(c)}₁₂ and {tilde over (c)}₂₂ are the codewords sent by S2.

In the non-cooperative configuration, as shown in FIG. 16, Node S1 andS2 transmit their own additional coded bits in turn during the 3^(rd)and last time slot for additional redundancy. For incremental bits, S1sends (c₁₂,c₂₂) while for joint network and channel coding, S1 sends(c₃).

8.2 MARC Topology

FIG. 10 shows S1 and S2, communicating with a BS through a RS usingOFDMA. Orthogonal subcarrier sets are assigned to the two users.

The proposed cooperative incremental redundancy space-time-coded relaytransmission may need two time slots to complete one cooperation cycle.

8.2.1 Time Slot 1

S1 transmits punctured FEC-coded and OFDM modulated sequences, where theFEC coded-modulation rate is R_(S) ₁ _(R) according to Equation 19:R _(S) ₁ _(R)≦α₁ I(SNR_(S) ₁ _(R) ⁽¹⁾)=R ₁ ⁽¹⁾  (19)α₁ is the fraction of time slot 1. When TDMA is used for user multipleaccess, α₁ denotes the fraction of time user 1 occupies in slot 1.8.2.2 Time Slot 2

S2 transmits punctured FEC-coded and OFDM/SCCP modulated sequences,where the FEC coded-modulation rate is R_(S) ₂ _(R) according toEquation 20:R _(S) ₂ _(R)≦α₂ I(SNR_(S) ₂ _(R) ⁽¹⁾)=R ₂ ⁽¹⁾  (20)α₂ denotes the fraction of time slot 2 used for transmitting S2'sinformation. For frequency division-based orthogonal multiple accesssuch as OFDMA, SC-FDMA and DFT-Spread-OFDM, α₁=α₂. For TDMA, α₂ denotesthe fraction of time that S2 occupies in slot 2. R_(S) ₁ _(R) and R_(S)₂ _(R) may or may not be the same. The BS and RS collect the receivedsequence. The RS will also decode the information sequence from S1 andS2. The BS then computes and stores the LLR.

8.2.3 Time Slot 3 and 4

The RS re-encodes the information sequences of S1 and S2 with theiroriginal rate-compatible FEC. The RS then maps the punctured coded bitsto symbols, and then uses OFDMA to modulate the modulated symbols of thetwo users.

S1 and S2 will also produce the same codeword and map the puncturedcoded bits to the same symbols as that of the RS, and then to theassigned subcarriers for OFDM transmission processing. Then S1, S2, andthe RS transmit the signals simultaneously to the BS, using Alamouticoding scheme.

The overall rate region is given by Equation 21:R ₁≦α₁ I(SNR_(S) ₁ _(D) ⁽¹⁾)+α₃ I(SNR_(S) ₁ _(D) ⁽²⁾+SNR_(RD) ⁽²⁾)=R ₁⁽²⁾R ₂≦α₁ I(SNR_(S) ₁ _(D) ⁽²⁾)+α₄ I(SNR_(S) ₂ _(D) ⁽²⁾+SNR_(RD) ⁽²⁾)=R ₂⁽²⁾  (21)where α₃ and α₄ are the fraction of resource which S1 and S2 used,respectively. The overall achievable rates are given by Equation 22:R ₁≦min(R ₁ ⁽¹⁾ ,R ₁ ⁽²⁾),R ₂≦min(R ₂ ⁽¹⁾ ,R ₂ ⁽²⁾)  (22)

However, if multiple turbo code is used with joint network-channelcoding, STBC cannot be employed. The rate region is similar to that ofthe CMAC.

8.2.4 Rate Setting

Select a target rate for S1 and S2, while assuming values for theSNR_(S) ₁ _(D) and SNR_(S) ₂ _(D). The values of SNR_(S) ₁ _(D) andSNR_(S) ₂ _(D) may for example be estimated through the process ofchannel estimation. If the channel is found to have a transmission ratethat is sufficiently high, a direct transmission mode may be used. Forexample, if both SNR_(S) ₁ _(D) and SNR_(S) ₂ _(D) are above apredetermined threshold, a direct transmission mode may be used.

For given values of SNR_(S) ₁ _(R) ⁽¹⁾ and SNR_(S) ₂ _(R) ⁽²⁾, we canselect α₁ and α₂ such that reliable transmission is possible on bothchannels. The slot duration should be minimized, i.e., using high-ratecoded modulation, so as to improve the overall efficiency based on twoconsiderations. The first consideration is, for decode-forward scheme towork, the RS needs to be close to the source nodes; The secondconsideration is, the transmission in these two slots do not have anyspace-time diversity at the BS, whereas the second slot sequences have.

With α₃, α₄ and the rate region, we can look for a minimum SNR_(RD) ⁽²⁾threshold which satisfies the target rate. If SNR_(RD) ⁽²⁾ is too low,we will have to lower our target rate and start all over again. Once therate is determined, a FEC scheme can be chosen to approach this rate.

8.2.5 Receiver Processing

The decoding process is similar to that for the relay network.

8.2.6 Training for Channel Estimation

Training signals can be transmitted in both time slots. In this case,constant-modulus training signals can be used by the S nodes in timeslot 1 with which the S-RS channel estimates can be obtained andconstant-modulus training signals can be used by the RS in time slot 3with which the RS-BS channel estimates can be obtained.

Alternatively, we can choose to transmit training signals only in timeslot 3. In this case, orthogonal training sequences need to be usedbetween the S and the RS in the respective subcarriers from which theS-BS and the RS-BS channel coefficients can be obtained.

8.2.7 Time and Frequency Synchronization Concurrent transmissions shouldbe aligned within the CP at the receiving node so as to maintainsubcarrier orthogonality; S1, S2 and RS should also use the same LOreference, e.g., the LO reference of the BS, for easier frequencysynchronization at the D or the R.

8.2.8 Transmission Protocol

Two source nodes S1 and S2 work in cooperation with the RS to delivertheir packets to the BS. In the 1^(st) time slot, node S1 transmitswhile node S2 transmits in the 2^(nd) time slot. The R receives thecoded bits sent by both nodes and attempts to decode their informationas shown in FIG. 19. For incremental bits, the node S1 sends thecodeword (c₁₁,c₂₁). For joint encoding, the node S1 sends the codeword(c₁,c₂). If decoding is successful, then this information will berelayed to the destination BS. ACK/NACK information is sent out from theRS to indicate whether they are cooperating or not in the 3^(rd) timeslot.

In the 3^(rd) time slot, cooperation occurs when the relay sends an ACK.Otherwise, S1 and S2 will operate in a non-cooperative mode. In thecooperative configuration that is shown in FIG. 21, the information isdecoded correctly. The RS can encode both information bit streamsseparately and transmit the additional coded bits with the correspondingsource together with a STBC, or the RS can jointly encode bothinformation bit streams and transmit to the BS.

If the RS fails to decode information from either of S1 or S2, then S1and S2 will operate in a non-cooperative mode. In FIG. 20, Nodes S1 andS2 transmit their own additional coded bits in turn during the 3^(rd)and last time slot for additional redundancy in the non-cooperativeconfiguration. Similarly, for incremental bits, S1 sends (c₁₁,c₂₂) andfor joint encoding, S1 sends (c₃).

8.3 Broadcast Relay System

FIG. 10 shows a downlink scenario where the BS communicates with twousers, S1 and S2 through a RS using OFDMA.

The proposed cooperative incremental redundancy space-time-coded relaytransmission may use two time slots to complete one cooperation cycle.

8.3.1 Time Slot 1

To communicate with S1, BS transmits a punctured FEC-coded and OFDMmodulated sequence, where the FEC coded-modulation rate is R₁. The rateis set such that the BS can decode the data correctly with a highprobability, according to Equation 23:R ₁≦α₁ I(SNR_(BR) ⁽¹⁾)  (23)where α₁ is the fraction of time slot 1 and SNR_(BR) ⁽¹⁾ is the SNR fromthe BS to the RS.

To communicate with S2, the BS transmits a punctured FEC-coded and OFDMmodulated sequence, where the FEC coded-modulation rate is R₂. The rateis set such that the BS can decode the data correctly with a highprobability. According to Equation 24:R ₂≦α₂ I(SNR_(BR) ⁽¹⁾)  (24)where α₂ denotes is the remaining fraction of time slot 1.

S1, S2 and the RS use the received sequences for decoding. The RS willdecode both the information sequences meant for S1 and S2. S1 and S2will then compute and store the LLR.

8.3.3 Time Slot 2

The BS and the RS will both transmit concurrently as follows. The RSre-encodes the information sequences of S1 and S2 with their originalrate-compatible FEC. The RS then maps the punctured coded bits tosymbols and then OFDMA modulates the modulated symbols of S1 and S2.

The BS will also produce the same codeword and maps the punctured codedbits or new coded bits to the same symbols as that in the RS, and thento the assigned subcarriers for OFDM transmission processing. The BS andthe RS then transmit the signals simultaneously to S1 and S2, using anAlamouti coding scheme.

The rate region for time slot 2 is given by Equation 25:R ₁≦α₁ I(SNR_(BS) ₁ ⁽¹⁾)+α₃ I(SNR_(BS) ₁ ⁽²⁾+SNR_(RS) ₁ ⁽²⁾)R ₂≦α₂ I(SNR_(BS) ₂ ⁽¹⁾)+α₄ I(SNR_(BS) ₂ ⁽²⁾+SNR_(RS) ₂ ⁽²⁾)  (25)where α₃ and α₄ respectively are the fraction of resources in which User1 and User 2 will use. The overall achievable rates for both slots areconstrained by the right-hand side of the above inequalities. Thusaccording to Equation 26:R ₁≦min(α₁ I(SNR_(BR) ⁽¹⁾),α₁ I(SNR_(BS) ₁ ⁽¹⁾)+α₃ I(SNR_(BS) ₁⁽²⁾+SNR_(RS) ₁ ⁽²⁾))R ₂≦min(α₂ I(SNR_(BR) ⁽¹⁾),α₂ I(SNR_(BS) ₂ ⁽¹⁾)+α₄ I(SNR_(BS) ₂⁽²⁾+SNR_(RS) ₂ ⁽²⁾))  (26)

8.3.4 Transmission Protocol

The BS works in cooperation with RS to deliver its packet to S1 and S2.In the 1^(st) time slot, the BS transmits, as shown in FIG. 22. Forincremental bits, node S1 sends the codeword (c₁₁,c₂₁). For jointencoding, node S1 sends the codeword (c₁,c₂). The RS receives the codedbits and attempts to decode its information. If decoding is successful,this information will then be relayed to S1 and S2. Alternatively if itfails, the BS will operate in a non-cooperative mode. The ACK/NACKinformation is sent out from the RS to indicate whether the nodes arecooperating or not in the 2^(nd) time slot.

In the 2^(nd) time slot, cooperation occurs when the RS sends an ACK.Otherwise, S1 and S2 will operation in a non-cooperative mode. For thecooperative configuration in FIG. 24 where the information is decodedcorrectly, the BS and the RS jointly transmits additional codewords(c₁₂,c₂₂) or (c₃) to both destinations using a STBC.

In the non-cooperative configuration illustrated in FIG. 23, the BStransmits additional coded bits during the 2^(nd) and the last timeslots for additional redundancy. The codeword (c₁₂,c₂₂) is sent if anincremental bit is used, and the codeword (c₃) is sent when jointencoding is used.

The hardware such as the ICs, UE (eg: S1 and S2), RS, BS, the centraloffice and other network equipment may be programmed with software tooperate according to one or more of the example embodiment methods, andotherwise compatible with common standards such as 3G, pre4G and/or 4G.These standards are incorporated herein by reference.

Whilst example embodiments of the invention have been described indetail, many variations are possible within the scope of the inventionas will be clear to a skilled reader.

In this specification, the terms “user”, “user equipment” (or itsabbreviation “UE”), node S1 and node S2 are to be interpreted asequivalents. In some network topologies such as CMAC, other users mayact as a RS, and RS and R are to be interpreted accordingly.

What is claimed is:
 1. A method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme, wherein determining the scheme includes selecting an additional coding technique from the group consisting of distributed turbo coding, enhanced turbo coding and multiple turbo coding.
 2. The method in claim 1 wherein the selection is based on which additional coding technique has a rate region that includes the predetermined transmission rate.
 3. The method in claim 1 wherein the determining whether to use distributing coding is made by determining if the SNR from the S to D is too low to support direct transmission.
 4. The method in claim 1 wherein the decode and forward relaying relaying space-time block coded data uses rate 1 convolutional or recursive convolutional codes with puncturing patterns.
 5. A method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme wherein the network topology is selected from the group consisting of a relay network, a CMAC network, a MARC network and a BRC network.
 6. A method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme wherein the determined scheme includes a plurality of slots in the time domain, a first slot being used by S to transmit a code word, if R can decode the code word then in a subsequent slot the relaying coded data is done cooperatively by the S and R.
 7. The method in claim 6 wherein the code word is relayed using a STBC.
 8. A method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme wherein a code word is optimized based on the channel SNR.
 9. A method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme using ODMA, TDMA, OFDMA, SC-FDMA, DFT-Spread-OFDM or SDMA.
 10. A RS configured to communicate with a plurality of UE according to a method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if the it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme, wherein determining the scheme includes selecting an additional coding technique from the group consisting of distributed turbo coding, enhanced turbo coding and multiple turbo coding.
 11. A RS configured to communicate with a BS according to a method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme, wherein determining the scheme includes selecting an additional coding technique from the group consisting of distributed turbo coding, enhanced turbo coding and multiple turbo coding.
 12. A BS configured to communicate with a plurality of UE according to a method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme, wherein determining the scheme includes selecting an additional coding technique from the group consisting of distributed turbo coding, enhanced turbo coding and multiple turbo coding.
 13. A communications network configured to communicate according to a method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme, wherein determining the scheme includes selecting an additional coding technique from the group consisting of distributed turbo coding, enhanced turbo coding and multiple turbo coding.
 14. A UE configured to communicate with a BS according to a method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if the it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme, wherein determining the scheme includes selecting an additional coding technique from the group consisting of distributed turbo coding, enhanced turbo coding and multiple turbo coding.
 15. An integrated circuit or processor including stored instructions, the instructions when executed control communication between a BS and a UE according to a method of communication comprising: determining whether to use a distributed coding between a S, R and D, based on a predetermined transmission rate; if the it is determined to use the distributed coding, determining a forward error correction scheme for the distributed coding, wherein the scheme includes distributed Alamouti space-time coding, and wherein the scheme is determined based on the predetermined transmission rate, a channel SNR and a network topology; and decode and forward relaying space-time block coded data from the S to the D using the determined scheme, wherein determining the scheme includes selecting an additional coding technique from the group consisting of distributed turbo coding, enhanced turbo coding and multiple turbo coding. 